1. Field of the Invention
The present invention relates to photographic lenses and, more particularly, a photographic lens comprising three lens components in total with a stop behind the lens system. Still more particularly, the invention relates to a photographic lens of back-stop type whose angle of view is widened to about 70 degrees, while still permitting aberrations to be corrected well over the entire area of the image frame, with a short total length of the entire lens system, suited to still cameras for the lens shutter, or video cameras.
2. Description of Related Art
Heretofore, for small-sized cameras such as a lens-shutter camera, there are widely used a triplet-type photographic lens comprising, in order from an object side, a positive lens, a negative lens and a positive lens, because relatively-good optical constants can be obtained by a small number of lens elements.
In the triplet type of photographic lens, there is a one that has a stop disposed behind the lens system. The use of such a back-stop type of photographic lens leads to make it relatively easy to reduce the size of the entire lens system. Further, the mechanical mounting for the lens becomes simpler in structure, and the focusing can be adjusted accurately and reliably. So, this type has been widely used in cameras.
Although the triplet-type photographic lens enjoys the advantages described above, when it is designed to widen the angle of view, the triplet-type photographic lens tends to produce large astigmatism in the marginal zone of the image frame.
For this reason, Japanese Laid-Open Patent Application No. Sho 59-34510, for example, has made a proposal that, for all the lens elements, glass materials of high refractive indices are selected to sustain the good optical performance in the marginal zone.
However, the glass materials of high refractive indices are expensive and their handling is difficult.
On this account, Japanese Laid-Open Patent Application No. Hei 1-133016 and Japanese Patent Publications No. Hei 7-60213 and No. Hei 7-92542 have made an alternative proposal of utilizing plastic material in manufacturing some of the components of the photographic lenses, since this material is amenable to relatively easy production techniques by injection molding.
Besides these, the use of the triplet form in the back-stop type photographic lenses is disclosed in, for example, Japanese Patent Publications No. Sho 48-5494 and No. Sho 50-2807.
Of the photographic lenses proposed in the above-cited references, a photographic lens proposed in Japanese Patent Publications No. Hei 7-60213 and No. Hei 7-92542 has its front or first lens element made up by using a glass material of high refractive index. Therefore, the outer diameter of the first lens element becomes larger than those of the second and third lens elements. Further, as the radius of curvature of the front surface has to be small, other problems arise in that large aberrations are produced and that the size of the entire lens system tends to increase greatly.
In a photographic lens proposed in Japanese Laid-Open Patent Applications No. Hei 1-133016, an aspheric surface is used in combination with the glass material of high refractive index in the rearmost or third lens element of positive refractive power. This photographic lens is, however, as narrow in the angle of view as 60 degrees, and the total length of the entire lens system tends to become relatively long.
A photographic lens proposed in Japanese Patent Publications No. Sho 48-5494 and No. Sho 50-2807 is relatively narrow in the angle of view, being 55 degrees. In addition, the ratio of the distance between the first lens surface and the image plane to the focal length, i.e., the so-called "telephoto" ratio, is more than 1.1, so that there is a tendency to have a relatively long total length of the entire lens system.